In a machine tool, a plurality of feed shafts are provided, which are driven by a linear motor and a servo motor. A feedback control is executed on each of the feed shafts, in which in order that the actual position of a driven object, such as at table on which a workplace is fixed, corresponds with a command position, a position detector is used to detect the position of the driven object so as to correct the error between the command position and the detected position of the driven object. In the feedback control, even though an unknown disturbance is input, a driving force is controlled so as to cancel out the disturbance. However, upon detecting the error, a driving force is input according to this error, and therefore there is a problem of a delay in response of the feed shaft.
The influence of a friction force, which is a type of the disturbance force, on the accuracy of contouring motion has been well known. For example, in a case of motion along an arc path using two shafts perpendicular to each other on the X-Y plane, each of the two shafts is given a sine-wave root ion command with a 90-degree phase shift. At the point at which the quadrant of the arch changes over to another quadrant, the motion direction of either one of the feed shafts is reversed. At this time, the direction of a friction torque and a friction force is also reversed, which are generated at a contact portion such as a ball screw or a bearing that is a constituent element of the feed shaft. Therefore, the control system for the reversing shaft responds with a delay of a given time. This causes a following error on a response path. The actual path passes along the slightly outer side of a command path. This phenomenon is referred to as “quadrant error”, which is a cause of reduction in motion accuracy.
A friction torque of a rotational system, and a friction force of a linear-motion system can be equivalently converted by a constant determined from the configuration of a mechanical system. Therefore, in the specification of the present invention, a friction torque and a friction force are not distinguished from each other. Also, a motor thrust of a linear-motion motor and a motor torque of a rotational motor are not distinguished from each other.
In order to solve the problem of degradation of the motion accuracy caused by the influence of a disturbance including a friction, there has been a known model-based disturbance correction method. In this method, a disturbance is estimated using a model, and a correction force that is required to cancel out the influence of the disturbance is input to a control system for a motor. For example, there is a method as disclosed in Patent Literature 1, to output a current correction command in a pulse waveform having a predetermined width for a predetermined time upon detecting a reversal of the motion direction. For another example, there is a method, disclosed in Patent Literature 2, to output a correction force expressed as a non-linear function to which the position of a driven object is input. In Patent Literature 3, two friction models are used in combination, which are a Coulomb friction, and a viscous friction that is proportional to the velocity.
In a device having a model-based disturbance correcting function, it is necessary to measure in advance a disturbance force generated in the device, and identify a model parameter to be used for the correction. In Patent Literature 1, based on the motion-error amount measured in advance, a pulse width and a pulse time of a correction command are determined. In Patent Literature 2, a relation between the motor current and the position at the time of reversing the motion direction is identified as a single-input single-output function. In Patent Literature 3, an identification algorithm is used to identify two constants that are a Coulomb-friction coefficient and a viscous-friction coefficient.
As disclosed in the following patent literatures, in a case where there are less factors of a disturbance model to be identified, or where a linear model is used, it is possible to identify a parameter of the disturbance model by means of relatively simplified measurement.